R600: Do not mergevector after a vector reg is used

R600: Do not mergevector after a vector reg is used

If we merge vector when a vector is used, it will generate an artificial
antidependency that can prevent 2 tex/vtx instructions to use the same
clause and thus generate extra clauses that reduce performance.

There is no test case as such situation is really hard to predict.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187516 91177308-0d34-0410-b5e6-96231b3b80d8